1. Field of the Invention
The invention is directed to a process for preparing isocyanates in the gas phase and in particular to improved mixing of the reactants used in such a process.
2. Description of the Prior Art
EP-A 0 289 840 describes a process for preparing (cyclo)aliphatic diisocyanates by phosgenation of the corresponding, gaseous (cyclo)aliphatic diamines at from 200° C. to 600° C. Phosgene is introduced in a stoichiometric excess. The superheated streams of, firstly, gaseous (cyclo)aliphatic diamine or (cyclo)aliphatic diamine/inert gas mixture and, secondly, phosgene are fed continuously into a cylindrical reaction chamber, mixed with one another there and reacted. The exothermic phosgenation reaction is carried out with turbulent flow being maintained.
Gaseous starting materials are frequently reacted in tube reactors. In the case of the jet mixer principle (Chemie-Ing.-Techn. 44 (1972) p. 1055, FIG. 10), two feed streams A and B are fed into the reactor, with feed stream A being introduced via a central nozzle and feed stream B being introduced via an annular space between the central nozzle and the wall of the tube reactor. The flow velocity of the feed stream A is high compared to the flow velocity of the feed stream B. As a result, the mixing of the reactants and consequently the reaction between them occur in the tube reactor. This way of carrying out the reaction has achieved industrial importance in the preparation of aromatic diisocyanates by phosgenation of aromatic diamines in the gase phase (e.g. EP-A-0 570 799).
The known processes require very long reactors since mixing occurs slowly without additional measures.
A consequence of the slow mixing of the reactants is the formation of polymeric by-products which lead to caking and even blockages in the reactor and thus shorten the operating period of the reactors. In addition, the greater lengths of the reactors lead to increased capital costs.
It is therefore an object of the invention to find a process for preparing (cyclo)aliphatic and aromatic diisocyanates by phosgenation of corresponding (cyclo)aliphatic and aromatic diamines in the gas phase at high temperatures, in which mixing of the reactants occurs significantly more quickly than in the processes known hitherto.